1. Field of the Invention
The present invention relates to a network system in which a plurality of multicast-supported networks are connected via gateway devices to a multicast-unsupported L2 (Layer 2) network, and to a gateway device for connecting the multicast-supported network to the multicast-unsupported L2 network.
2. Description of the Related Art
Over the recent years, a plurality of networks (such as networks extending in the same enterprise and over a plurality of offices of an organization) have been connected to each other via a multicast-unsupported L2 network such as a wide area Ethernet network. In the case of simply connecting the multicast-supported networks to each other via the wide area Ethernet network etc., however, it follows that a multicast packet is to be broadcast over the wide area Ethernet network. Then, if the multicast packet is broadcast over the wide area Ethernet network, there arise problems, wherein bands of the Ethernet network are used with futility, the multicast packet is transmitted also over a network having no necessity of transmission, and so on. Therefore, on the occasion of connecting the multicast-supported networks to each other via the wide area Ethernet network etc., as schematically shown in FIG. 26, there is provided a server that translates a packet format by temporarily deassembling the packet back into video data etc., and the multicast packet is translated by encapsulation into the unicast packet as schematically shown in FIG. 27.
Further, as schematically shown in FIG. 28, the multicast-supported networks are connected by preparing a dedicated line (and two relay devices) between every two networks that require relay of the multicast packet.
When the multicast-supported networks are connected in the configurations (topologies) as illustrated in FIGS. 26 through 28, it follows that there do not arise any problems derived from broadcasting the multicast packet over the wide area Ethernet network. In a system (FIG. 26) that terminates a multicast protocol by an application program, however, an extra period of time corresponding to the time needed for processing in each server is expended in transferring the video data etc.
Further, the packet transfer between the multicast-supported networks in the configuration as shown in FIG. 27 must involve preparing a device having a function of transmitting the packet in a way that segments the packet and a function of reassembling the packet transmitted in an as-segmented form. The reason is that a packet size after the encapsulation is not necessarily a transmission-enabled size over the wide area Ethernet network. Then, the device having these functions can not be manufactured at a low cost, and hence it takes a considerably high cost to transfer the packet between the multicast-supported networks in the configuration as shown in FIG. 27. Moreover, on the occasion of connecting the multicast-supported networks in the system (topology) shown in FIG. 28, there is also the necessity of preparing the dedicated line (and the two relay devices), and the considerably high cost is required.